Natural gas consists primarily of methane, approximately greater than ninety mole percent (>90 mol %). Raw natural gas extracted from a well commonly exists in mixtures with other hydrocarbons, principally ethane, propane, butanes, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H2S), carbon dioxide, helium, nitrogen, and small quantities of other compounds.
Natural gas processing involves separating all of the various hydrocarbons and fluids from the raw natural gas, which is primarily methane, to produce what is known as “pipeline quality” dry natural gas. The owners of major interstate transportation pipelines, as well as local distribution companies, usually impose restrictions on the make-up of the natural gas that is allowed into the pipeline. Therefore, natural gas is typically purified before it is transported from its origin to remove, e.g., to remove water, hydrogen sulfide, and carbon dioxide. As a term of art in the natural gas industry, “pipeline quality” gas has come to mean natural gas that (1) has a heating value within five percent (+/−5%) of the heating value of pure methane, or 1,010 Btu per cubic foot under standard atmospheric conditions, and (2) is free of water and toxic or corrosive contaminants. See The Natural Gas Industry: Evolution, Structure, and Economics, Arlon R. Tussing and Bob Tippe, Pennwell Books, 1995. However, each pipeline operator individually determines the applicable quality specifications based on the particular supply sources, operational constraints, and end user requirements. See Interstate Natural Gas—Quality Specifications & Interchangeability, Michelle Michot Foss, Center for Energy Economics, Institute for Energy, Law & Enterprise, University of Houston Law Center (December 2004) available at http://www.beg.utexas.edu/energyecon/Ing/documents/CEE_Interstate_Natural_Gas_Quality_Specifications_and_Interchangeability.pdf. There is not a single specification that applies to all natural gas pipelines, although most pipelines have similar quality requirements and all must meet certain baseline requirements to ensure safe pipeline operation and comply with government regulations. Therefore, as used herein, “pipeline quality” natural gas means gas that meets the general range of gas specifications giving rise to this requirement within the U.S. gas transportation industry.
While certain amounts of the ethane, propane, butanes, and pentanes typically are removed from natural gas, this does not mean that they are all “waste products.” In fact, those associated hydrocarbons are typically very valuable by-products of natural gas processing and can be sold separately. Those hydrocarbons have a variety of different uses, including enhancing oil recovery in oil wells, providing raw materials for oil refineries or petrochemical plants, and as sources of energy. For example, the liquefied petroleum gas (“LPG”) used in home heating, cooking, and transportation engines is a combination of propane and butanes obtained from natural gas processing. However, the actual practice of processing natural gas to pipeline dry gas quality levels is quite complex and energy intensive.
Natural gas streams require a number of pre-processing and processing steps before the components, e.g., dry natural gas and Natural Gas Liquids (“NGL”), attain specifications that permit their transport and sale as commercially viable products. Large classes of natural gas wells can generate processed gas streams in which the ethane content is too high to recover optimal value of the dry natural gas product, yet too low to justify economically its removal from the natural gas product. In some of these cases, NGL pipelines can safely and effectively transport the heavier hydrocarbons (e.g., propane and butanes), but cannot safely transport gaseous ethane streams. In these cases the ethane becomes stranded at the gas processing or well-source location. With limited or no attractive economical purification, transport, or use options, the only viable option is to burn the natural gas stream for fuel value. A process that can remove the problems presented by stranded ethane gas within natural gas source areas will provide economic and potential environmental advantages.
Additionally, one option for natural gas processing is to convert certain constituents obtained from natural gas into olefins, which have a variety of uses. This is typically accomplished by the non-catalytic thermal or steam cracking of saturated hydrocarbons, such as ethane and/or propane and/or butanes, or the catalytic dehydrogenation of ethane and/or propane and/or butanes. A typical ethylene plant using steam cracking of ethane achieves an ethylene selectivity up to about 80-85% calculated on a carbon atom basis at a per pass ethane conversion of about 65-60%, respectively. But steam or thermal cracking processes, as well as catalytic dehydrogenation processes, for olefin production are highly endothermic. Therefore, these processes consume a large amount of energy and require large, capital-intensive, and complex cracking furnaces to supply the requisite heat. Steam or thermal cracking processes also typically produce a large number of by-products, including propylene, butenes, higher olefins, diolefins, benzene, toluene, xylenes, and acetylenes, as well as higher alkanes and other higher hydrocarbons. Therefore, cracking processes require extensive refining to separate the by-products into useful components for sale or manufacture of other products. Currently, over 90% of the ethylene produced in the world is derived from naphtha steam cracking, with the balance of about 10% being from ethane, propane, and butane steam cracking.
Naphtha steam cracking is a highly endothermic process, requiring large energy input and large, capital-intensive, and complex cracking furnaces. Naptha steam cracking also produces a variety of products other than ethylene, including propylene, propane, butenes, butanes, higher olefins, diolefins, benzene, toluene, xylenes, and acetylenes. These byproducts are costly to separate from ethylene, which is usually done by cryogenic distillation, extractive distillation, and/or selective hydrogenation to the corresponding mono-olefin, e.g. acetylene to ethylene.
Steam cracking facilities, with the supporting infrastructures and processes, are among the most complex projects in the petrochemical industry, requiring a tremendous initial investment to construct a plant on an economically viable scale. Additionally, upstream refining, separation, and purification processes, e.g., distillation or adsorption, are typically capital and energy intensive processes. Storing and transporting high purity hydrocarbon feedstock, particularly gases like ethane, presents substantial environmental and safety risks, requiring further capital and energy intensive processes. In order to make a steam cracking process capital efficient, it is typically conducted on an extremely large scale, e.g., a typical plant may produce about 2 billion pounds of ethylene per year.
There are existing processes that utilize purified hydrocarbon feedstock, particularly ethane, derived from natural gas. Those processes typically require a high purity feedstock (e.g., approximately greater than ninety-nine percent (>99%)) that involves upstream refining, separation, purification, and supply infrastructure. The processes for converting a high purity, natural gas-derived hydrocarbon feedstock, particularly ethane, to a more valuable commodity chemical, particularly ethylene, acetic acid, ethylene oxide, and/or vinyl acetate monomer (“VAM”), do not facilitate or incorporate value from the back-integrated use of impure hydrocarbon feed streams, such as ethane derived from the processing of natural gas. Some processes have been described for some limited integrated production of ethylene, acetic acid, and VAM from high purity ethylene, but those processes similarly do not facilitate or incorporate value from the back-integrated use of impure ethane feed streams. In addition, even the limited integration described does not fully utilize all of the possible forward or downstream process integration.
For example, U.S. Pat. Nos. 4,250,346; 4,524,236; 4,899,003; and 5,162,578, which are incorporated herein by reference, disclose ethane oxydehydrogenation (“EODH”) processes whereby ethane and an oxidant, typically oxygen or air, is converted into ethylene and/or acetic acid. EODH is an exothermic process and, therefore, requires diluents to control the heat produced during the reaction. Typically, the best diluents are those gases with a high heat capacity. For example, U.S. Pat. Nos. 5,155,262; 5,183,936; and 5,198,578, which are incorporated herein by reference, demonstrate the effects of diluent heat capacity on the performance of catalytic oxidation processes, particularly the conversion of propylene and isobutylene to acrolein and methacrolein, respectively.
The existing EODH processes are also not able to take full advantage of the exothermic nature of the EODH process, because they do not allow for downstream integration. Some of those processes are exclusively focused on producing ethylene from ethane and, thus, do not enable the benefits of integration with downstream and upstream processes, which can benefit from the heat generation. Similarly, those processes that lack downstream integration do not benefit from the advantageous generation of acetic acid as a by-product of the EODH process. Likewise, some processes focus on producing acetic acid from ethane and, thus, do not enable the benefits of integration with upstream and downstream processes, such as those that benefit from the advantageous generation of ethylene. Some of those processes enable the co-production of ethylene and acetic acid from ethane, but they do not enable the benefit of back-integration for upstream use of methane and other high heat capacity diluents from impure ethane-containing natural gas streams.
EODH is only one type of a catalytic oxydehydrogenation (“ODH”) process that is specifically applied to ethane, but other alkanes can also be converted to alkenes and alkene derivatives by ODH processes. For example, U.S. Pat. Nos. 5,639,929; 5,780,700; 5,902,918; 6,072,097; 6,235,678; 6,355,854; 6,576,803; 6,756,517; 6,846,773; 6,858,768; and 7,067,455, which are incorporated herein by reference, disclose ODH processes for converting hydrocarbons into olefins. Those processes suffer from the same deficiencies as the previously described EODH processes, specifically they require feedstocks that have been previously purified to remove the non-reactant alkanes and other impurities. Therefore, the previously described processes do not enable backward (or upstream) integration with raw natural gas streams and they are not integrated with forward or downstream processes.
Vinyl acetate monomer (VAM) is manufactured from the oxidative reaction of ethylene and acetic acid. U.S. Pat. No. 5,821,384 and the like describe processes for converting ethylene and acetic acid in the presence of an oxidant and a catalyst into VAM. Furthermore, U.S. Pat. Nos. 6,040,474; 6,605,739; 6,040,474; and 7,491,843 describe processes for the integration of an EODH process for manufacturing ethylene and acetic acid with a downstream process for manufacturing VAM. However, those processes do not enable the benefits of integration of further downstream and upstream processes, particularly utilization of impure natural gas feedstocks and further derivatization or polymerization of VAM. In addition, those processes do not integrate the removal of carbon oxides and their subsequent utilization in further downstream processes.
Ethylene and vinyl acetate may be copolymerized to prepare a range of useful products that vary in properties and performance, depending on the ratio of ethylene to vinyl acetate in the polymer. Ethylene vinyl acetate (“EVAc”) copolymer is manufactured by the free radical polymerization of VAM and ethylene at high pressure and temperatures. As used herein, EVAc is meant to represent any and all copolymers of ethylene and vinyl acetate, regardless of the ethylene-vinyl acetate ratio. Previously described processes for producing EVAc and its copolymers do not include the integration of ethane-containing feedstocks (e.g., unpurified natural gas), ethylene production, VAM production, or the ability to derive these products alone from ethane-containing feedstocks. In addition, those processes do not include the ability to integrate with upstream processes for producing ethylene, VAM, and/or acetic acid, such as the EODH process. Particularly useful integration steps disclosed herein include, without limitation, carbon dioxide utilization, heat and energy utilization, and compression.
There is a need for a process that enables the use of ethane to capture higher value products while integrating by-product utilization and energy recovery to provide lower cost and reduced environmental impact.